1. Field of the Invention
The present invention is directed to an improved internal combustion engine for increasing fuel efficiency while reducing exhaust emissions and a method of operating such an engine. In particular, the present invention is directed to such an engine operable in a premixed charge compression ignition
2. Description of Related Art
Relatively recently, because of the increased regulatory pressure for fuel efficient and low emissions engines, some engine designers have directed their efforts to one type of an internal combustion engine which utilizes premixed charge compression ignition (PCCI). Researchers have used various other names in referencing PCCI combustion including homogeneous charge compression ignition (HCCI) as well as others such as xe2x80x9cATACxe2x80x9d which stands for xe2x80x9cActive Thermo-Atmosphere Combustion.xe2x80x9d (SAE Technical Paper No. 790501, Feb. 26-Mar. 2, 1979), xe2x80x9cTSxe2x80x9d which stands for xe2x80x9cToyota-Sokenxe2x80x9d (SAE Technical Paper No. 790840, Sep. 10-13, 1979), and xe2x80x9cCIHCxe2x80x9d which stands for xe2x80x9ccompression-ignited homogeneous chargexe2x80x9d (SAE Paper No. 830264, 1983). All of these terms are hereinafter collectively referred to as PCCI.
Generally, conventional internal combustion engines are either a diesel or a spark ignited engine, the diesel engine controlling the start of combustion (SOC) by the timing of fuel injection while a spark ignited engine controls the SOC by the spark timing. Initially, it should be understood that SOC refers to the point in time at which a charge within the cylinder begins to ignite. The major advantage that a spark ignited natural gas or gasoline engine has over a diesel engine is its ability to achieve extremely low NOx and particulate emissions levels. Premixed charge spark ignited engines have nearly homogeneous air fuel mixtures which tend to be either lean or close to stoichiometric, resulting in very low particulate emissions. The major advantage that diesel engines have over premixed charge spark ignited engines is in its higher thermal efficiency. Typical diesel engines, however, cannot achieve the very low NOx and particulate emissions levels which are possible with premixed charge spark ignited engines.
Another type of engines that has been recent focus of research and has been proposed and studied is direct injection natural gas engines that utilizes compression ignition. In such engines, highly pressurized natural gas is injected directly into the combustion chamber during or after compression so that the heat generated by compression ignites the injected natural gas in a manner similar to that of diesel injection applications. Such direct injection natural gas engines allow higher compression ratios than spark ignition natural gas engines. Hence, the gross thermal efficiency of direct injection natural gas engines is known to be higher than that of spark ignition natural gas engines. However, direct injection natural gas engines require the natural gas to be compressed to very high pressures such as 3000 psi or greater which is very difficult to attain. This required compression process requires substantial amount of work which reduces the brake thermal efficiency of direct injection natural gas engines. Consequently, whereas the emission performance in direct injection natural gas engines have been found to be better than conventional diesel engines, the higher emissions (as compared to spark ignited engines) as well as complexity and high cost of such engines have minimized their commercial appeal.
Unlike the above described internal combustion engines, engines operating on PCCI principles rely on autoignition of a relatively well premixed fuel/air mixture to initiate combustion. More specifically, in PCCI engines, the fuel and air are mixed in the intake or in the cylinder, long before ignition occurs. The extent of the mixture may be varied depending on the combustion characteristics desired. Some engines may be designed and/or operated to ensure that the fuel and air are mixed into a homogeneous, or nearly homogeneous, state. Also, an engine may be specifically designed and/or operated to create a somewhat less homogeneous charge having a small degree of stratification. In both instances, the mixture exists in a premixed state well before ignition occurs and is compressed until the mixture autoignites. Thus, PCCI combustion event is characterized in that: 1) the majority of the fuel is sufficiently premixed with the air to form a combustible mixture throughout the charge at the time of ignition; and 2) ignition is initiated by compression ignition. In addition, PCCI combustion is also preferably characterized in that most of the mixture is significantly leaner than stoichiometric to advantageously reduce emissions, unlike the typical diesel engine cycle in which a large portion, or all, of the mixture exists in a rich state during combustion. Because an engine operating on PCCI combustion principles has the potential for providing the excellent fuel economy of the diesel engine while providing NOx and particulate emissions levels that are much lower than that of current spark ignited engine, it has also recently been the subject of extensive research and development.
It is now known that for efficient, low emission PCCI combustion, it is important to have the combustion event occur at an appropriate crank angle during the engine cycle. In this regard, it has further been found that the timing of SOC and the combustion rate (therefore combustion duration) in a PCCI engine primarily depend on various combustion history values such as the temperature history; the pressure history; fuel autoignition properties (e.g. octane/methane rating or activation energy); and trapped cylinder charge air composition (oxygen content, EGR, humidity, equivalence ratio etc.). However, it should be understood that the term PCCI does not exclude the use of ignition timing mechanisms such as pilot injections and spark ignition known in the art that are used to precisely time the ignition of the premixed charge. Whereas the premixed charge may combust due to compression, such ignition timing mechanisms aid in initiating the SOC of the premixed charge at a precise time to ensure desirable combustion characteristics. This is in contrast to non-PCCI engines such as conventional gasoline engines with spark ignition in which the premixed charge of gasoline and air would not ignite at all without the spark.
A premixed charge compression ignition engine with optimal combustion control with various control features for controlling SOC and the combustion rate is disclosed in the patent application Ser. No. 08/916,437 filed on Aug. 22, 1997, currently assigned to the Assignee of the present invention. This application has also been published as International Patent Application No. PCT/US97/14815. As disclosed in the ""437 application, active control is desirable to maintain the SOC and duration of combustion at the desired location of the crankshaft and at the desired duration, respectively, to achieve effective, efficient PCCI combustion with high efficiency and low NOx emissions. In this regard, the ""437 application discloses a PCCI engine comprising a combustion history control system that includes at least one of a temperature control system for varying the temperature of the mixture of fuel and air, a pressure control system for varying the pressure of the mixture, an equivalence ratio control system for varying an equivalence ratio of the mixture and a mixture autoignition property control system for varying an autoignition property of the mixture.
The engine uses an operating condition detecting device that detects an engine operating condition and provides a corresponding signal to a processor that generates one or more control signals to control the combustion history control system such as the temperature control system, the pressure control system, the equivalence ratio control system and/or the mixture autoignition property control system. In this manner, variable control of the combustion history of future combustion events may be attained. A start of combustion (SOC) sensor such as a cylinder pressure sensor may be used to sense the start of combustion so that effective feedback control may be also attained.
Due to the difficulties in controlling the timing of the SOC in a PCCI combustion event, variations of engines operating in a PCCI mode have been proposed including internal combustion engines that operate in a dual fuel mode in which the engine operates in a conventional diesel mode, a PCCI mode, or a combination thereof. For instance, the ""437 application further discloses such an engine which may be operated in different modes and the use of an injector to inject additional gas or liquid such as diesel fuel into the cylinder to time the PCCI combustion event. It should be understood that the term PCCI does not exclude the use of ignition timing mechanisms such as injections and spark ignition known in related art that are used to precisely time the ignition of the premixed charge. Whereas the premixed charge would combust due to compression, such ignition timing mechanisms aid in initiating the SOC of the premixed charge at a precise time to ensure desirable combustion characteristics.
In this regard, the ""437 application discloses that the diesel fuel may be injected either early in the compression event or later in the compression event near top dead center (TDC) to initiate a PCCI combustion event. Thus, the late injection adds a small amount of stratified fuel which is compression ignited to help control the ignition of the premixed fuel during the PCCI combustion event. Such early injection has been found to be advantageous in that it provides an effective way to initiate and control ignition of the premixed charge and to control SOC in a PCCI engine.
In addition, other PCCI type engines have been proposed in which a pilot injection of diesel fuel is directly injected into the premixed charge late in the combustion cycle such as near top dead center to ignite the premixed charge to thereby allow precise timing of the SOC. However, in contrast with true PCCI engines in which sufficient premixed charge is provided to allow autoignition of the premixed charge due to compression, these types of engines do not provide the proper conditions (such as gas temperature and pressure near TDC) for autoignition to occur. Thus, without the pilot injection which acts as the ignition source by providing a flame front (or some other ignition mechanism), no ignition of the premixed charge would occur. Thus, these types of engines do not combust the premixed charge in a true PCCI event and are somewhat similar to conventional gasoline engines with spark ignition in which the premixed charge would not ignite at all without the spark or other flame propagation mechanism.
Despite these significant recent developments in the technology of internal combustion engines, there still exists an unfulfilled need to further increase the fuel efficiency of internal combustion engines while minimizing exhaust emissions. These further improvements to exhaust emissions are desirable and necessary to ensure meeting the ever increasingly stringent government emissions requirements, especially with respect to NOx emission levels which has been difficult to reduce further using the presently known technology and methods.
In view of the foregoing, it is an object of the present invention to provide an internal combustion engine and method that operates in a premixed charge compression ignition mode with increased fuel efficiency.
Another object of the present invention is to provide such an engine and method that reduces exhaust emissions.
Still another object of the present invention is to provide such an engine and method which allows sufficient early control injection to be injected into the combustion chamber to properly control start of combustion while minimizing possibility of fuel spray impinging on the combustion chamber walls.
In accordance with one embodiment of the present invention, these objects are attained by an internal combustion engine operable in a premixed charge compression ignition mode and a method for controlling the engine, the engine comprising an engine body with a piston assembly, a combustion chamber formed in the engine body by the piston assembly, an intake system for delivering intake air to the combustion chamber during an intake stroke, a mixing device that mixes a first fuel with the intake air to provide a premixed charge of air and the first fuel, a direct fuel injector adapted to directly inject a second fuel into the combustion chamber, and a control system adapted to control the direct fuel injector in a manner to provide at least one early control injection of the second fuel into the combustion chamber before start of combustion of the premixed charge. The early control injection of the second fuel is preferably injected into the combustion chamber during at least one of the intake stroke and a compression stroke during which the premixed charge is compressed in the combustion chamber. In one embodiment, the early control injection of the second fuel may be injected into the combustion chamber during the intake stroke to allow sufficient mixing of the second fuel with the intake air. The mixing device may include at least one of a carburetor, a throttle body injector, and a port fuel injector that is adapted to mix air and the first fuel upstream of the combustion chamber.
In accordance with various embodiments of the present invention, the control system may be adapted to control at least one of injection timing, injection duration, injection rate and injection amount of the early control injection. The control system may also be adapted to variably control the amount of the second fuel injected during the early control injection by the direct fuel injector based on at least one of an operating condition and operating mode of the internal combustion engine. The timing and amount of the early control injection may be variably controlled based on desired timing of start of combustion of the premixed charge.
In accordance with another embodiment, the at least one early control injection is less than 10 percent of the fuel to be combusted and preferably less than 5 percent. At least one of the first fuel and the second fuel may be at least one of diesel, natural gas, gasoline, propane, naphtha, and kerosene. In one embodiment, the first fuel and the second fuel are different types of fuels.
In another preferred embodiment, the at least one early control injection is a plurality of early control injections. In this regard, each of the plurality of early control injections are less than 10 percent of the fuel to be combusted, and preferably less than 5 percent of the fuel to be combusted. In another embodiment, each of the plurality of early control injections are separated by a time gap sufficient to allow mixing of a control injection prior to beginning another control injection and/or sufficient to allow the direct injector to recover from a control injection prior to beginning another control injection. In this regard, each of the plurality of early control injections may be separated by a time gap. The time gap between each of the plurality of early control injections is preferably sufficient to allow mixing of an early control injection prior to beginning another early control injection and/or sufficient to allow the direct fuel injector to recover from an early control injection prior to beginning another early control injection. The time gap may be between 500 xcexcsec and 2500 xcexcsec, preferably approximately 1250 xcexcsec.
In still another embodiment, the engine body includes a plurality of cylinders each having a piston assembly that forms a combustion chamber, and the control system is further adapted to vary the amount of the second fuel injected by the direct fuel injector during the plurality of early control injections fewer than all of the plurality of cylinders at a time.
These and other objects, features and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments of the present invention when viewed in conjunction with the accompanying drawings.